The present invention relates to an electrophotographic color image recorder and, more particularly, to an image transferring device incorporated in such a recorder for transferring toner images to a recording medium one above another.
A digital color copier, for example, belongs to a family of electrophotographic color image recorders extensively used today. A digital color copier has an image reading section and an image forming section. In the image reading section, optics illuminates a document and then optically separates it into red, blue and green components. These color components each is converted into a digital signal by a particular CCD (Charge Coupled Device) array. The digital signals from the CCD arrays are processed and then applied to semiconductor lasers corresponding one-to-one to the CCD arrays as image data. In the image forming section, laser beams issuing from the lasers are incident to photoconductive elements each being assigned to a particular color component, thereby electrostatically forming latent images on the photoconductive elements. Developing units each storing respective one of a yellow toner, magenta toner, cyan toner and black toner develop the latent images by the toners. A belt carrying a recording medium in the form of paper sheet thereon sequentially transports the paper sheet to successive image transfer positions defined below the photoconductive elements. The toner images of different colors are sequentially transferred from the photoconductive elements to the paper sheet one above another by transfer chargers. The resulted composite color image formed on the paper sheet is fixed by a fixing unit. Then, the paper sheet carrying the fixed color image thereon is driven out onto a tray.
As stated above, the belt and transfer chargers incorporated in the color copier constitute an image transferring device. It has been customary to use backup members which urge the belt against the photoconductive elements from the rear of the belt in order to enhance the close contact of the paper sheet and the photoconductive elements. Usually, the backup members are implemented as metallic rollers which contact the rear of the belt and support the belt while being rotated by the latter. However, the problem is that as the belt made of a dielectric substance and charged by the transfer chargers are abruptly brought into and out of contact with the metallic rollers, undesired electric fields are developed therebetween. Such undesired electric fields disturb the desired electric fields developed between the transfer chargers and the photoconductive elements. In addition, when the belt leaves any one of the rollers, the potential therebetween increases to cause a separation discharge to occur with the result that the charge on the rear of the belt is neutralized, loosing the force for retaining the toner. Consequently, the toner images to be transferred to the paper sheet are disturbed. Regarding the disturbance to the images, a reference will be made to Japanese Patent Laid-Open Publication Nos. 33072/1980 and 97357/1981.
In light of the above, a plurality of pairs of field stabilizing plates may be arranged along opposite surfaces of the belt at positions other than the positions where the photoinductive elements face the associated transfer chargers with the intermediary of the belt, i.e., at positions between nearby photoconductive elements and between nearby transfer chargers. Each of the filed stabilizing plates intervening between the transfer chargers, i.e., located at the rear of the belt has a lug at the center thereof which abuts against the rear of the belt. Such field stabilizing plate pairs eliminate undesired electric fields and prevent the potentials from increasing, thereby freeing images from disturbance. However, the corona discharge effected by the transfer chargers produce nitrogen oxides, while the belt and a drive roller driving the belt produce low-resistance impurities such as carbon of the drive roller since they slide on each other. As such impurities deposit between the belt and the backup lugs, the electric fields fluctuate in the vicinity of the belt to cause discharges to occur, disturbing the toner image. Furthermore, when the belt is supported by the backup lugs, the pressure urging the belt, i.e., the paper sheet against any one of the photoconductive elements is not always optimal and, moreover, susceptible to ambient conditions. If this pressure is low, the toner image will not be satisfactorily transferred and, therefore, the image density on the paper sheet will be low. Conversely, excessively high pressures would cause thin lines and the edges of solid images each having a substantial width, especially the portions thereby where a great amount of toner deposits, to be lost inside. While a toner with an additive which enhances fluidity may be used to eliminate such an occurrence, the amount of additive should be limited in consideration of the filming to occur on the photoconductive elements and the change in ambient conditions.
An implementation for using a paper sheet against the photoconductive elements by an adequate pressure is disclosed in Japanese Patent Laid-Open Publication No. 127770/1987 by way of example. This implementation uses a presser member in the form of a thin elastic plate made of a dielectric material such as polyethylene terephtharate. The presser member presses the belt against the photoconductive element at a position immediately preceding an image transfer position, thereby causing a paper sheet into close contact with the photoconductive element. Subsequently, as soon as the paper sheet moves away from the image transfer position, the presser member releases the belt from the photoconductive element. The presser member is associated with the transfer charger. The presser member is successful in urging the paper sheet against the photoconductive element by an adequate pressure. However, since this approach has no means for stabilizing electric fields between nearby image transfer positions, it is likely that undesired electric fields are developed in the vicinity of the belt to disturb the toner image. An extra function is available with the presser member, i.e., a function of preventing the charge developed the transfer charger from extending to the zone upstream of the image transfer position. Specifically, the presser member may be arranged such that the transfer charger does not act on the zone upstream of the position of the belt which the free end of the pressure member contacts. Nevertheless, the position where the free end of the presser member contacts the belt is not always the optimal boundary of the zone of interest. Moreover, since the free end of the presser member contacts the photoconductive element via the belt and paper sheet, a spatial discharge is apt to occur between the presser member and the photoconductive element due to the transfer charge sequentially accumulating on the member, especially when humidity is low. The spatial discharge would disturb the toner image carried on the photoconductive element and the toner image transferred to the paper sheet.